Operation Free Sinjar aims to cordon off the town, take control of Islamic State supply routes and establish a buffer zone to protect the town from artillery, a statement from the Kurdish national security council said.
Sinjar is a symbolic and strategic prize, sitting astride the main highway linking the cities of Mosul and Raqqa – Islamic State's bastions in Iraq and Syria. US led coalition air strikes pounded Islamic State-held areas in the town overnight, as around 7,500 Kurdish special forces, peshmerga and Yazidi fighters descended from the eponymous mountain towards the frontline in a military convoy.
Kurdish forces and the U.S. military said the number of Islamic State fighters in the town had increased to nearly 600 after reinforcements arrived in the run-up to the offensive, which has been expected for weeks but delayed by weather and friction between various Kurdish and Yazidi forces in Sinjar.
The offensive is being personally overseen by Kurdistan regional President Massoud Barzani, who is also head of the Kurdistan Democratic Party (KDP), which other groups in the area accuse of seeking to monopolise power.
Many Yazidis lost faith in the KDP when its forces failed to protect them from Islamic State militants, who consider them devil worshipers, when the group attacked Sinjar in August 2014, systematically slaughtering, enslaving and raping thousands of them.
A Syrian affiliate of the Kurdistan Workers' Party (PKK) came to the rescue, evacuating thousands of Yazidis stranded on Sinjar mountain and establishing a permanent base there. Near the frontlines on Thursday, a Kurdish officer stood behind a wall of sandbags. Sinjar, about 300 metres (980 feet)away, could be seen through a gap in a rampart.

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